Hi-
Oops, you are using quite old version of Oscommerce.
In newer version you would edit the shipping.php class file, but the version you are using. You would edit the modules/shipping/item.php.
Just make the cheapest() return $shipping_cheapest = 'item';
That shall work for any shipping methods that you want to make default selected.
Go to the corresponding module file and modify the cheapest().
_
Thanks
kashif